How to expose the raw bytes in Python container classes to C functions using Cython.

README.rst

Accessing Raw Bytes in Container Classs Using Cython

Description

This gist demonstrates how to expose the raw bytes encapsulated by several Python container classes to a C function using Cython.

Usage

1. Make sure you have Cython installed.
2. Build the included extension by running make.
3. Run the demo file test.py.

License

This software is licensed under the BSD License by Lev Givon.

funcs.pyx

# How to expose a C function to various Python container classes.

cdef extern from "myfuncs.h":
    unsigned char _sum "sum"(unsigned char *, int)

from cpython.buffer cimport PyBUF_SIMPLE
from cpython.buffer cimport Py_buffer
from cpython.buffer cimport PyObject_GetBuffer

def sum(s):
    cdef Py_buffer buf
    PyObject_GetBuffer(s, &buf, PyBUF_SIMPLE)
    return _sum(<unsigned char *>buf.buf, buf.len)

Makefile

funcs:
	python setup.py build_ext --inplace

test: funcs
	gcc -L. test.c -o test -lmyfuncs

clean:
	rm -f test *.o *.so funcs.c

myfuncs.c

/* Simple shared library. */
#include "myfuncs.h"

unsigned char sum(unsigned char *x, int n) {
    unsigned char result = 0;
    int i;

    for (i=0; i < n; i++) {
        result += x[i];
    }
    return result;
}

myfuncs.h

unsigned char sum(unsigned char *x, int n);

setup.py

#!/usr/bin/env python

from distutils.core import setup
from distutils.extension import Extension
from Cython.Build import cythonize

extensions = [
    Extension('libmyfuncs',
              sources=['myfuncs.c'],
              headers=['myfuncs.h']),
    Extension('funcs',
              sources=['funcs.pyx'],
              libraries=['myfuncs'],
              library_dirs=['.'])
    ]
setup(ext_modules = cythonize(extensions))

test.c

/* Test of function provided by myfuncs library. */
#include <stdio.h>
#include "myfuncs.h"

int main(void) {
    unsigned char x[] = {1, 2, 3};
    unsigned char result;

    result = sum(x, 3);
    printf("%i\n", result);
}

test.py

#!/usr/bin/env python

# Test the extension:
import funcs
import numpy as np

a = np.array([1, 2, 3], dtype=np.uint8)
s = '\x01\x02\x03'
b = bytearray([1, 2, 3])

print funcs.sum(a)
print funcs.sum(s)
print funcs.sum(b)

a = np.array([1, 2, 3], dtype=np.uint32)
s = a.tostring()
b = bytearray(a)

print funcs.sum(a)
print funcs.sum(s)
print funcs.sum(b)

a = np.array([1, 2, 3], dtype=np.float64)
s = a.tostring()
b = bytearray(a)

print funcs.sum(a)
print funcs.sum(s)
print funcs.sum(b)

don't forget to PyBuffer_Release(&buf)